Fix bandwidthThrottling metrics for reads. The read stream was

incorrectly charging the entire buffer size against the throttler
rather than just the number of bytes actually read.

For example, if you did a read with a 32KB buffer which only received
300 bytes across the wire, the bandwidth Throttler would charge
you for a full 32KB of bandwidth rather than the 300 bytes that
were really used.  This was grossly overstating the bandwidth
consumption to the throttler.

@@ -58,9 +58,11 @@ static NSLock *readLock = nil;
 		}
 		[request performThrottling];
 	}
-	[ASIHTTPRequest incrementBandwidthUsedInLastSecond:toRead];
 	[readLock unlock];
-	return [stream read:buffer maxLength:toRead];
+	NSInteger rv = [stream read:buffer maxLength:toRead];
+	if (rv > 0)
+		[ASIHTTPRequest incrementBandwidthUsedInLastSecond:rv];
+	return rv;
 }
 
 /*